In a process of developing silver salt photographic materials, which may be color photographic materials or black-and-white photographic materials, a desilvering step follows a developing step, to remove silver and silver salts (hereinafter referred to simply as "silver") that have become unwanted in the photographic material, and it then passes into a processing solution.
Since the continuous processing of photographic material results in an accumulation of dissolved silver in the processing solution, ultimately the processing solution gradually loses its desilvering ability. Consequently, for economy, for the utilization of resources, or to comply with environmental protection regulations in some areas, it is required that silver is recovered from the silver-containing processing solution at a suitable stage, so that the processing solution can be made reusable (regeneration) or to make possible the processing of the waste solution. Further, BOD and COD of the waste solution is to be lowered to comply with environmental protection regulations.
Processing solutions capable of dissolving silver salts that are used in such a desilvering step are mainly fixing solutions (for color photographic materials and for black-and-white photographic materials) and bleach-fixing solutions (for color photographic materials). It is needless to say that since silver salts are carried by photographic materials in a washing bath or an image-stabilizing bath, which follows the desilvering step from the preceding bath, the washing bath and the image-stabilizing bath are also solutions from which silver is to be recovered. In this specification and claims, such processing solutions that contain silver and the waste solutions thereof are generally called "silver-containing solutions."
Various techniques for recovering silver from silver-containing solutions have long been known, and typical techniques are classified broadly into a metal-substitution process, a electrolytic recovery process, a chemical settling process, and an ion exchange resin process, which are further improved in various ways depending, for example, on the form of the developing process, properties of the photographic material, and regional circumstances.
The present invention relates, in particular among these, to an improvement in the electrolytic recovery process. As apparent from U.S. Pat. No. 1,876,830, the electrolytic recovery process is a method that has long been known and is in wide practical use. The advantages thereof are, for example, that chemical processing procedures are not required, that non-destructive recovery is possible, that is, changes of components other than silver can be kept small, thereby enabling a thus-treated solution to be advantageously used again as a processing solution, that since silver can be recovered in the form of a metal, recovered silver can be easily refined and highly purified, and that the electrolytic apparatus is simple and is small-sized. On the other hand, the electrolytic recovery process has such disadvantages that (1) the electrolysis speed is slow, (2) the recovering rate of silver is low, (3) if the silver-containing solutions came, in particular, from color processing, the recovering rate and the electric current efficiency are low due to the adulteration of iron salts. To correct the disadvantages, a method of electrolysis wherein silver is recovered not as metallic silver but as silver sulfide, that is, in which the applied voltage is kept high, is suggested and described in Research Disclosure (RD) No. 13702 (September 1975). In this regeneration method, sulfide ions (S.sup.2-) are produced in an electrolytic chamber containing an aqueous solution of thiosulfate salts, and then they are transferred into a reaction chamber charged with a silver-containing waste solution to convert the silver to silver sulfide. The method is different from the present invention because the present invention contemplates to electrolize the silver-containing waste solution directly.
U.S. Pat. No. 1,937,179 discloses a method wherein silver is collected as silver sulfide (usually as silver) by electrolytic collection to allow a fixing solution to be reused. Thus, this method is different from the present invention with respect to the object, the procedure, and the conditions (such as applied electritic voltage) since electrolysis is conducted in the method substantially without causing any influence on the thiosulfate salts contained in the fixing solution to allow the fixing solution to be reused. Since this is a method for recovering part of dissolved silver and for reusing a fixing solution, the mode of this method is also different from that of the present invention.
However, although the method for recovering silver electrolytically as silver sulfide has improved electrolysis speed, it has the following disadvantages:
(1) The silver sulfide precipitate is in a state of suspended fine particles, which are difficult to separate. PA1 (2) Since, in addition to the electrolytic cell, an independent reactor and settling tank are required, the apparatus becomes complicated and its cost is high. PA1 (3) The current efficiency is low. This is regarded as being due to various secondary electrolytic reactions that take place simultaneously.